274 Dr. C. R. A. Wright on the Determination of 



It is noticeable that whilst experiments Nos. I. and II. show 

 that the curves obtained are by no means necessarily identical 

 even when the conditions are sensibly the same (owing appa- 

 rently to differences in the character of the copper deposited 

 during the action of the cell), experiments Nos. I., III., and IV. 

 indicate that but little difference in the curve is brought about 

 by using commercial instead of pure zinc, or by altering the 

 surface of the zinc by covering the bright metal with a film of 

 copper (although more or less marked depressions in the maxi- 

 mum E.M.F. are occasioned thereby); on the other hand, 

 experiments V. and VII., as compared with the others, indi- 

 cate that amalgamating the copper renders the rate of fall in 

 E.M.F. sensibly more rapid. But little difference, on the 

 whole, is apparently occasioned in the curves by the use of 

 zinc-sulphate solution (whether stronger or weaker than the 

 copper-sulphate solution), as compared with dilute sulphuric 

 acid ; what difference is brought about is of this kind — that 

 the sulphuric-acid curves slightly underlie the zinc-sulphate 

 curves. 



In none of the experiments made was any measurable de- 

 pression of the E.M.F. of the cell brought about when the 

 current flowed at a rate not exceeding 8 microamperes per 

 square centimetre; and in several cases this rate of flow might 

 be doubled before any depression greater than '001 volt (0*1 

 per cent.) was occasioned. As a rule, when the current- 

 density was from 30 to 50 microamperes per square centimetre, 

 a diminution in the E.M.F. of from 0*5 to 1 per cent, was 

 brought about; whilst diminutions of 10 per cent, and upwards 

 were occasioned when the current-density exceeded 3000. 

 Supposing the same values to hold for ordinary Daniell cells 

 (which is probably not quite the case, as the zinc and copper 

 plates are usually unequal in size), it results that, with cells of 

 ordinary dimensions (e. g. holding a litre and exposing a sur- 

 face of 500 square centimetres), no appreciable diminution in 

 the E.M.F. would be brought about when the current does 

 not exceed 500x8 = 4000 microamperes (-004 ampere); 

 whilst diminutions of several tenths per cent, would be occa- 

 sioned with currents of fivefold magnitude (-02 ampere), 

 and diminutions of 10 per cent, and upwards when the cur- 

 rent exceeds 500x3000 = 1,500,000 microamperes (1 *5 am- 

 pere). 



